This invention refers to embankment dams, and to an improved method for their construction and waterproofing.
Water resources become more and more precious and their conservation is becoming more and more important; therefore it is essential to search for and adopt solutions which minimise the waste of water and which allow a clever management of the existing water resources.
The most ancient typology of a dam is the embankment dam, obtained by using natural materials available on site for the creation of embankments capable of contrasting the pressure exerted by the water collected in the natural reservoir delimited by the dam itself. The dam body must be statically stable and at the same time it must avoid water leakage caused by possible infiltration, which would cause a decrease of the quantity of water resource available, and which could also jeopardise the stability or the safety factor of the dam itself. As a matter of fact, uncontrolled water infiltration into the dam body can cause undesirable interstitial pressures, erosion phenomena and the formation of preferential flows or of xe2x80x9cpipingxe2x80x9d capable of causing even the collapse of the whole structure.
In many cases earthfill and/or rockfill dams are preferred to conventional concrete dams, to roller compacted concrete (RCC) dams, to masonry dams or other, as they are less expensive; therefore it is important to build embankment dams which have a high safety factor and are watertight.
During the years different techniques have been developed to make embankment dams watertight. There are substantially two tendencies for the waterproofing of embankment dams: the first one consists in waterproofing the upstream face, and the second one consists in creating a waterproofing core inside the body of the dam itself.
The waterproofing of the upstream face stops possible infiltration onto the dam surface next to the water impounded in the reservoir. The waterproofing barrier is executed on the slopes of the dam body and is therefore subject to stresses and deformations which occur over time in the dam body. This kind of barrier must therefore have good characteristics of elasticity and at the same time or watertightness.
In general this kind of barrier consists of an upstream face built in concrete, with waterproofing joints, waterstops In synthetic material and/or copper, or with facings made of a bituminous concrete.
In both cases the deformations which the dam body undergoes during exploitation are such as to cause possible failures in these waterproofing barriers with subsequent water loss and risk for the stability of the structure.
Recently, watertight upstream facings have been executed with flexible synthetic geomembranes, capable of granting the watertightness of the dam and at the same time capable of sustaining strong deformations, even concentrated, without damage.
Geomembranes simply laid over the upstream face of the dam, however, need ballasting layers in order to avoid that the geomembrane itself can be displaced or damaged by the suction exerted by winds, or by the fatigue caused by the action of waves.
A second solution which has been widely adopted in construction of embankment dam foresees the construction of a central watertight care, made with natural materials positioned so as to grant low permeability, lower than 1xc3x9710xe2x88x9210 cm/sec, for example clay or bentonite, placed during construction of the embankment. In the latest decades, the central core has also been constructed in bituminous concrete and in conglomerates cement-bentonite based.
All the above-mentioned solutions have emphasised some constructive difficulties, as well as a rather high probability not to be able to reach the required reliability, besides the impossibility of checking the extent of their efficiency by measuring the occurring seepage. Furthermore, should infiltration or water leakage through the central core occur, the repair is extremely difficult and brings uncertain results.
An embankment dam of the type mentioned above is described in DE-A-4.402.862; this document suggests also the use of a water lightening core in bituminous concrete, and a sealing membrane to provide a small cavity upstream the central core and a filtering material, to allow said cavity to be filled with water upon construction of the dam, to subject the same dam to the maximum hydrostatic condition in absence of water into the basin
Type and nature of the membrane is not described or suggested in this document, because the waterproofing of the dam is performed by the bituminous concrete core. Furthermore DE-A-4.402.862 does not suggest or make obvious to gradually construct the membrane and a transition zone of loose material, during construction of the dam, as well the use of a fine loose material suitable to inject a sealing substance upon failure of the membrane.
Hence, for the construction of embankment dams, the need of finding new construction and waterproofing solutions which, by using artificial materials, allow to obtain an effective watertightness for the whole life of the dam, by using systems and materials which can be coupled with aggregates of the dam body capable of granting only the static function, and which are also easily and economically constructed, whose efficiency can be checked over time and which, in case of damage, can be simply and efficiently repaired.
The main object of this invention is to execute an embankment dam and a construction and waterproofing method, which can reach the above-mentioned objectives, allowing to make consistent savings on the total cost of construction of the dam.
In particular, an object of this invention is to supply a method for the construction and the waterproofing of an embankment dam which uses a watertight barrier capable of adapting to any deformation of the dam body without loosing its efficiency or its watertightness.
A further object of this invention is to supply an embankment dam and construction and waterproofing method which allow to adopt suitable monitoring systems of the watertightness of the waterproofing barrier, and which at the same time allow to intervene for the necessary repairs, or to execute waterproofing connections with other rigid structures of the dam itself.
Another object of this invention is to supply a method for the construction and the waterproofing of an embankment dam which allows to use an upstream waterproofing system comprising a proper flexible synthetic geomembrane extending from the crest to the upstream toe of the dam, allowing a non-rigid connection of the geomembrane itself, capable of following the deformations, sometimes high, of the dam body itself which can occur over time.
A further object of this invention is to supply a method for the construction and the waterproofing of an embankment dam which allows to immediately use the dam, even if not yet finished, during its construction.
According to this invention, a method has been provided for the waterproofing of a dam comprising an embankment body in coarse loose material having a longitudinal axis, in which the aforementioned dam body is made by superimposing layers of earth and/or rocks, and a water proofing barrier comprising a waterproofing membrane and a transition zone of fine loose material which develop from the bottom towards the top and which extend along the longitudinal axis of the dam, characterised by performing the waterproofing barrier including at least one synthetic and elastically yeildable waterproofing geomembrane and disposing at least on transition zone of selected loose material on at least the downstream side of the geomembrane, said loose material having a high water permeability for the injection, of fluid or fluidised sealing materials, gradually performing the transition zone and the waterproofing membrane during the construction of the body of the dam body; and providing anchoring means for progressively anchoring the waterproofing geomembrane to the transition zone during the construction of the dam.
According to a first particular aspect of the invention a method has been provided for the construction and the waterproofing of darns for retaining water in a reservoir, in which the dam comprises a body in coarse loose material, made by superimposed layers in earth and/or rock or similar, to provide a static function to resist to the thrust impounded by the water in the reservoir, the method comprising the steps of performing a central core defining a water-barrier in selected fine loose material, from sand to gravel, with a high permeability, higher than the dam body, for instance comprised between 1xc3x9710xe2x88x921 and 1xc3x9710xe2x88x925 cm/sec; this barrier incorporates at least a waterproofing membrane in an elastically yieldable synthetic material, extending from the dam body foundation to the crest, and longitudinally to the dam itself. At least one side of the waterproofing membrane is covered by at least one layer of synthetic material, for example a geotextile, capable of protecting the membrane against the mechanical aggression of the inert loose materials of the central core; the waterproofing membrane and the protecting layer in synthetic material being progressively incorporated in the different superimposed layers in loose material, during the construction of the dam body and of the central core.
According to another aspect of the invention, a method has been provided for the construction and the waterproofing of dams designed for retaining water in a reservoir, in which the dam comprises a body in coarse loose material, made of superimposed and compacted layers in earth and/or rock or similar, and a waterproofing membrane in an elastically yieldable synthetic material, extending from the dam body foundation to the crest, and longitudinally on an upstream face of the dam, the waterproofing membrane being fastened by means of strips of said elastically yieldable synthetic material, previously embedded between superimposed layers of loose material of said body of the dam, and successively welded to the waterproofing membrane during the construction and the installation on the above-mentioned upstream face.
According to a particular aspect of the invention, the waterproofing membrane is built on the upstream face of the dam already completed, by adjoining several sheets in synthetic material which are unrolled from the top to the bottom of the dam and welded to the anchoring strips embedded in the dam body during its construction.
According to another particular aspect of the invention, the waterproofing membrane is built on the upstream face of the dam by adjoining several sheets in synthetic material which are laid horizontally in respect to the longitudinal axis of the dam and welded to anchoring strips in synthetic material embedded in the dam body during the construction of the dam itself. This solution is particularly advantageous if compared to the previous ones because it allows an anticipate, although partial use of the dam, during its construction, without having to wait for the long time normally required for the stabilsation and testing of the dam after completion.
In the solution with the waterproofing membrane laid directly on the upstream face of the dam, the use of a waterproofing synthetic material, flexible and elastically extensible, coupled and adherent to a substrate in if synthetic material, such as a geotextile or similar, besides supplying a mechanical protection against any accidental puncturing of the waterproofing membrane by the loose material of the dam, supplies also surface with a high friction coefficient. This surface with high friction coefficient allows to maintain in their position the single sheets of the membrane during their installation, even if they are not yet welded to the anchoring strips.
The connection between the anchoring strips and the waterproofing membrane can be executed by thermo-welding in accordance with specific methods further explained, by using in any case for the waterproofing membrane and for the anchoring strips, synthetic materials that are chemically compatible for their heat-welding.
According to one more aspect of the invention, the lower edge of the waterproofing membrane is fastened to the dam body""s upstream toe by creating a longitudinal bend which allows the membrane itself to better adapt to possible movements of the dam body.
For the scope of this invention the various words used have the meaning herein defined:
geomembrane: flexible synthetic material with two prevailing dimensions, characterised by a low permeability to fluids;
geocomposite: flexible synthetic material with two prevailing dimensions, made by coupling, during production, of two or more layers of synthetic materials with different characteristics and functions, one of which consists of a geomembrane having a waterproofing function;
geosynthetic: synthetic material with two prevailing dimensions; which depending on its characteristic can have different functions such as waterproofing, antipuncturing protection, sliding, etc.;
geotextile: synthetic material consisting of textile fibres, with high permeability;
layered membrane: consists of at least two layers of synthetic materials with two prevailing dimensions, having different functions, which can be coupled during manufacturing or can be only superimposed during the construction of the dam.